1. Field of the Invention
The present invention relates to machines useful for moving a tool or similar implement into contact with a workpiece. More particularly, the invention relates to a method and apparatus including a micropositioner for positioning the tip of an ultrasonic or similar wire bonding tool at precisely determinable locations relative to a microcircuit or similar workpiece, moving the tip into contact with the workpiece to form a bond, locking the tip against motion in at least one direction during formation of the bond, and withdrawing the tip upon completion of a bond.
2. Description of Background Art
The invention of the transistor in 1948 may well have been the most important technological development of the 20th century. Substitution of transistors for vacuum tubes made it possible to construct computers of substantially reduced size and cost, much greater speed and data handling capability, and greatly reduced power consumption than vacuum tube computers. In the latter part of the 1950's, transistors were introduced to a wide variety of military, industrial and consumer products.
In the early 1960's, integrated circuits were introduced. Integrated circuits contain a number of transistors and interconnecting circuit elements which together comprise an operable electronic circuit, such as a logic element, amplifier, or memory element. While the first integrated circuits contained just a few transistors, integrated circuits are now being manufactured which contain tens or even hundreds of thousands of transistors on a single semi-conductor die or chip.
Integrated circuits are fabricated from thin slices of a semi-conducting material such as silicon, germanium or gallium arsenide. The slices are cut into small squares or rectangles referred to as chips or dice, ranging in size from squares about 100 mil. (0.100 inch) on a side to several hundred mils. on a side. Transistors, diodes, resistors and interconnecting circuit paths are formed on the chip or die by diffusing impurities into selected regions of the chip, and by depositing various conducting paths and insulating layers onto the chip.
After a semi-conductor chip or die has been fabricated as described above, it must be attached to a base or carrier forming part of a package or container to protect the delicate die from damage. Prior to packaging, conductive pads providing input and output ports to the die must be electrically interconnected to more robust leads or terminals which extend outward through a container or package used to enclose the die. These interconnections are customarily made using fine aluminum or gold wires which are ultrasonically or thermosonically welded to the pads and leads by a bonding tool that applies ultrasonic energy, or a combination of heat and ultrasonic energy, to a bonding site. Since the connection pads of a microcircuit are extremely tiny and closely spaced, great precision is required in positioning the tip of a bonding tool relative to the microcircuit.
In response to a perceived need for an apparatus capable of precisely positioning the tip of an ultrasonic transducer used to form wire bonds on microcircuit chips, the present inventor invented a micro-positioner apparatus employing a novel pantograph-type manipulator input mechanism. That apparatus which was disclosed in the inventor's U.S. Pat. No. 3,474,685, that issued on Oct. 28, 1969, has proven to be highly effective in performing its intended function, and wire bonding apparatus employing the novel design concepts of the pantograph mechanism disclosed in that patent are used widely throughout the industry. However, the present inventor found that certain aspects of the micro-positioner disclosed in the U.S. Pat. No. 3,474,685 patent might be improved upon. For example, the allowable working or throat depth of the ultrasonic tool tip would preferably be larger. Also, the use of offset pivotable mountings for the transducer tool plate would desirably be minimized, thereby minimizing the requirement for springs to counter balance unbalanced forces exerted in supporting the tool plate by offset mountings. Moreover, it would be desirable to have a micropositioner apparatus in which various ultrasonic transducers and other bonding tool accessories such as wire spooling mechanisms, some of which might be substantially heavier than conveniently supportable by prior art micropositioners, could be used. Such a need arises, for example, in bonding the heavier wires required for connection to various electronic components such as wound coils. In response to those considerations, the present inventor disclosed a Micropositioner For Ultrasonic Bonding, in co-pending U.S. patent application Ser. No. 08/773,637, filed Dec. 24, 1996.
However, in bonding insulated wires using ultrasonic transducers, an additional problem has existed with prior art bonding machines. Thus, when bonding an insulated wire to a microcircuit pad or device package lead, ultrasonic heating energy supplied by the tip of an ultrasonic bonding tool to form a bond must first melt insulation covering the wire prior to welding the wire to the pad or lead. Melting of bonding wire insulation typically requires about 300 milliseconds. This period is substantially longer than the 30-millisecond period typically required for welding bare wire. Therefore, even experienced operators of ultrasonic bonding machines, who have developed the requisite motor skills to position and maintain the tip of an ultrasonic bonding tool for a time interval sufficient to bond a bare wire to a bond site, tend to move the tip too soon after initiation of electrical drive power to a transducer to bond an insulated wire to a bond site. This premature movement can result in formation of a defective bond.
Although premature movement of the bonding tool tip in any direction can adversely affect ultrasonic bond formation, the present inventor has determined that premature motion in the up and down or z-axis direction is the most problematic, with longitudinal y-direction motion nearly as problematic, and motion in a lateral or x-direction somewhat less problematic. The present invention was conceived of to alleviate the aforementioned problems.